Method and drive system for the control/regulation of linear pressure/cast movement

ABSTRACT

The invention relates to a method for controlled driving of the cast axle ( 8 ) of pressure die casting machines, comprising a drive piston ( 15 ), wherein the forward and return motion of the piston ( 15 ) is carried out in a controlled manner. The invention is characterized in that the oil flow or oil pressure on the piston rod side is controlled/regulated by a pump ( 30 ) and additionally by a throttle valve ( 103 ) working in parallel at least during the filling phase, especially during the fast forward motion part.

TECHNICAL SCOPE

[0001] This invention relates to a method and a drive system for controlling/regulating the linear pressing/casting movement by means of a driving piston in injection molding machines and die-casting machines or presses, whereby a hydraulic medium is used at least on the drive side and/or the piston rod side.

STATE OF THE ART

[0002] In addition to the mold cavity, the compression axle or die-casting axle is essentially the heart of the die-casting machine or press. This is also true of cold or hot chamber magnesium die-casting machines. High injection forces and practical working casting rates of more than 3 m/sec to 5 m/sec are required. In the area of injection molding machines for use with plastics, machines that operate only with an electric drive have already become well established to a great extent, in addition to traditional machines driven by oil hydraulic systems. In cold and hot chamber magnesium die-casting machines, all the axles except the casting axle can be driven by electric motor. The casting rate here is even higher. In the case of machines with a closing force of more than 200 metric tons, it is practically not economically feasible to meet the extreme requirements of force and speed with the traditional technology unless it is based on a hydrostatic piston drive, as in the state of the art. With this novel invention, this fact must also be taken into account. This invention is therefore based on the use of a hydraulic piston for the pressing or casting axle. However, even in the case of a piston implementation, a limiting factor is the speed and thus the control/regulation of the speed program of the pressing or casting piston.

[0003] U.S. Pat. No. 4,022,269 discloses schematically a drive system for a die-casting machine, where the back side of the piston is acted upon by a compressed gas from two pressurized containers. The compressed gas acts through appropriate throttles and controlled/regulated valves. The compressed gas acts on the back side of the driving piston. However, a hydraulic fluid is used on the front side. On the one hand, pressure is generated by a pump, and on the other hand, it is released through a valve. A non-return valve is situated between the pump and the forward piston space and/or the space on the side of the piston rod. The reverse movement of the driving piston is accomplished directly by way of the hydraulic pump. In the actual working movement, compressed gas is used on the back side of the piston. The oil to be displaced on the piston rod side is discharged only through the valve because it is blocked by the non-return valve without a reverse flow through the pump.

[0004] The problems associated with the casting axle are described below on the basis of a few “textbook examples” of the state of the art. An entire die-casting machine consists of a mold closing unit 2 and a casting unit 3 (FIG. 1). The drive system 4 for a movable mold 5, which is attached to a mounting plate 7 by columns 6 in a non-positive manner, is situated on the side of the mold closing unit 2. The casting piston rod 8 is moved by a casting drive 10, and in the example illustrated here, an accumulator 9 is equipped with an extra pre-accumulator 11. The machine sits on a machine base 12.

[0005]FIGS. 2 and 2a show the casting unit 3 of a cold chamber machine with casting drive 10, shown schematically on an enlarged scale with the casting equipment (left), having the following essential components. Accumulator 9 is connected by a shot valve 13 to the back side 14 of the driving piston 15 to generate the required pressure for the three phases (FIG. 7): the forward pressure, the mold filling and the mold filling pressure. In the casting driving cylinder 16 the casting piston rod 8 is fixedly connected to the driving piston 15 on the piston rod end. On its other end, the casting piston rod 8 has a casting piston 17 and it projects into a casting chamber 18 into which molten metal 20 is fed through a filling port. The molten metal 20 is pressed by the forward movement according to arrow 21 through a cutout 22 into the mold cavity 23 of the two die-casting molds 24 and 25. The individual control functions are initiated by way of control means (not shown), especially for the shot valve 13, a hydraulic pump 30 and a control valve 31. Oil at the desired pressure is supplied through the control valve 31 by way of a line 32 for the return movement of the casting piston 17. In the forward movement, i.e., the actual casting operation, oil displaced by the movement of the driving piston 15 is drained into a tank 33 on the piston rod side, like the reflux of a pressure-limiting valve 34. FIG. 2 illustrates schematically how the casting drive is controlled/regulated with a continuous valve with the possibility of real-time digital control.

[0006]FIG. 3 shows a known example of a path-time diagram at a constant forward speed and at a constantly accelerated forward speed. The forward flow and of mold filling phases are clearly discernible. The constant forward speed can be seen as a straight line and the constantly accelerated forward speed is shown as a parabola.

[0007]FIGS. 4 and 5 show a more complicated casting drive with two piston-type accumulators 40, 41 and a multiplier 42. In this example according to FIG. 4, the pressure medium is always a hydraulic fluid 43.

[0008]FIG. 6 shows schematically an example of a hot-chamber casting drive. As also shown in FIG. 6, three phases may be differentiated in the casting cycle. For the first phase, which is the forward phase, the hydraulic medium is conveyed by the pump through a forward valve 44 and a return valve 45 to the back side 14 of the driving piston 15. For the second phase, which is mold filling, the piston-type accumulator I and/or 41 is switched on by a speed control valve 46 and a shot valve 47. For the third phase, the mold filling pressure, the piston-type accumulator II and/or 40 is activated by a control valve 48 and a multiplier valve and multiplier 42. The hydraulic pressure can be increased from 55 bar to 140 bar and/or to 210 bar by the three-stage operation according to this example.

[0009] The state of the art described here is explained in the technical book “Praxis der Druckgussfertigung” [Practice of Fabrication by Die-Casting] Ernst Brunhofer, Verlag Schiele & Schön, Berlin (1991 edition).

[0010]FIG. 12 shows a typical example of a press. In the state of the art, the speed of the plunger is limited like the speed of the casting piston.

EXPLANATION OF THE INVENTION

[0011] This invention is based on the object of finding a simple design for the drive for the pressing and/or casting axle, which will allow optimum control of the casting piston movement and an increase in the pressing/casting piston speed in comparison with the state of the art.

[0012] The method according to this invention is characterized in that the pressing/casting movement is controllable/regulatable with respect to the oil flow and/or oil pressure on the side of the piston rod by way of a pump and, at least in some phases, additionally by way of a throttle valve (34, 46) which operates in parallel with the pump.

[0013] The device according to this invention is characterized in that both a pump and also at least one parallel-connected throttle are arranged on the side of the piston rod such that at least for the area of rapid forward movement, both the flow through the pump and the flow through the throttle can be controlled/regulated at the same time by motor means.

[0014] This novel invention has four very important advantages: with the simultaneous use of the pump and throttle for the return flow, it is possible to increase the speed of the pressing/casting movement with a relatively small increased complexity in terms of the design. The pump may thus be designed to be as small as possible and the throttle may be designed to be as large as necessary. This yields a relatively advantageous structural complexity. The throttle discharge, as a path subject to a very high loss, is used extremely briefly, and throttle losses are therefore minimized. Reference is made in this regard to European Patent 0 782 671. The pump may be used for energy recovery by way of the drive motor in the return flow because the motor and the pump can function as a generator.

[0015] In the forward movement, the pump may be used as a motor, and the energy obtainable via the server motor may be used for other axles and/or a portion may be stored in DC capacitors. The entire system may be used entirely or partially in the sense of an oscillator due to the fact that the largest possible energy component is only shifted back and forth in the system and is not destroyed. This novel invention makes it possible to utilize the acknowledged advantages of an electric drive essentially on larger machines with a closing force of more than 200 metric tons through a novel combination of servo motor and hydraulic system. All previous considerations regarding better control of the pressing or casting axle are concentrated largely on the back side of the piston and not on the rod side of the piston, with the goal of having even higher pressures act on the piston even more rapidly. This novel invention does not preclude the corresponding additional expenditures in terms of construction and control technology but it does make it possible to prevent them in many cases, and it solves the problem on the other side of the pressing or casting side and/or the piston rod side. A more rapid discharge of hydraulic oil on the piston rod side, which is also more demanding from the standpoint of control technology, is ensured, particularly in the phase of the greatest piston speed required. In the phase that is usually critical, this results in two oil flows, which are controlled at the highest level of the control/regulating technology. This makes it possible to select relatively small units for the individual components because they support one another mutually.

[0016] The surprising advantage, e.g., with respect to a largely dissipation-free design or a design without control valves, is that a small energy loss due to a valve subject to dissipation must be accepted during less than {fraction (1/10)}th of a second per casting cycle under some circumstances. Due to the reciprocal movement of the hydraulic medium on the back side of the piston and due to alternating use of the drive motor on the piston rod side, the drive motor may be used as a motor for the pump or as a generator (for the remaining period of time), as proposed in International Patent WO97/05387 and in European Patent 0 782 671, for example. It is also important that even in the case of an implementation that is subject to dissipation on the passive side of the drive, it is possible to prevent a load alternation in the hydraulic pump or in the coupling between the motor and the pump when there is a change in the direction of rotation of the pump. The accuracy of the control/regulation for the pump side is increased significantly.

[0017] This novel invention permits a large number of advantageous embodiments. Reference is made in this regard to claims 2 through 7 and 9 through 15. An especially advantageous embodiment is characterized in that the piston is driven for the forward movement either directly or indirectly by a gas, in particular nitrogen (N₂) from a pressurized nitrogen tank through appropriate valves. The pump is designed as a fixed-volume pump, and the control/regulation of both the pump and the throttle valve is accomplished by way of a servo motor for each case. Both the pump and the throttle return flow are connected as a closed system to a closed and more or less pressureless oil container in the form of a pressure reservoir, corresponding to accumulator 9. A vacuum degassed oil is preferably used here. Traditional hydraulic oil, a chemical oil, or a mixture of water and glycol, for example, may be used as the hydraulic fluid. This novel invention offers the special advantage that it is possible to work with an extremely small amount of hydraulic fluid, e.g., on the order of 10 to 20 liters, whereas 100 to 200 liters are easily present in the system with the known designs. Another advantage is the extreme reduction in the risk of fire, thanks to the extremely small amount of hydraulic fluid.

[0018] A similarly advantageous embodiment of the device is characterized in that an essentially known piston-type accumulator is connected upstream from the driving piston on the back side (for the forward movement). A gas supply is accomplished here by way of a rapidly switching valve and a direct connection to a pressurized nitrogen container to the piston-type accumulator. It is also proposed that the pump be designed as a fixed-volume pump with a servo motor drive. The throttle also has a positioning motor drive, in particular a controlled/regulated servo motor, where the overdrive may be accomplished by way of a transmission, a spindle or a crank drive. In a preferred embodiment, the positioning drive has a spindle overdrive with a spindle nut, where the spindle nut is incorporated into the servo valve, and the servo motor, the spindle and the throttle form a compact structural unit.

[0019] The throttle is designed as a piston valve, where the throttle cross-section is adjustable directly by the servo motor and the spindle overdrive by means of a linear displacement. The drive system also has pressure gauges for measuring the injection force and a distance measuring system for the position of the pressing/casting piston as well as a control/regulating unit designed as a multi-variable control that detects in particular the position and/or speed and/or force of the driving piston.

BRIEF DESCRIPTION OF THE INVENTION

[0020] This novel invention is illustrated now on the basis of a few examples with additional details. They show:

[0021]FIG. 7 this novel invention in a purely schematic diagram;

[0022]FIG. 8 a somewhat more concrete embodiment of FIG. 7;

[0023]FIG. 9 the novel throttle valve, adjustable by motor;

[0024]FIG. 10 a pictorial/schematic diagram of a multi-variable control system;

[0025]FIG. 11 the profiles for various parameters of a casting operation;

[0026]FIG. 12 a press in which this novel invention is used for the movement of the press ram.

METHODS AND EMBODIMENT OF THE INVENTION

[0027] It can be seen in FIGS. 7 and 8 that the structural embodiment of the casting equipment is unchanged in comparison with the known designs. However, the elements for control of the oil flow on the piston rod side are designed according to a completely novel concept. A liquid medium, i.e., a hydraulic oil, is proposed as the medium on the piston rod side, circulating through a connecting line 108/111 between chamber 109 on the piston rod side of the casting driving cylinder 16 and a preferably closed tank 110. The oil flow passes through line 108 and is conveyed directly through a pump 30 and line 113. As indicated by the two arrows 114 and 15, the oil flow may go in one direction or the other as needed. Pump 30 is driven by a servo motor 105 and has the properties described in European Patent 782 671 and WO97/05387, for example. servo motors are characterized in particular by an electronic power unit 116 by which all the important parameters can be controlled virtually simultaneously via an on-site electronic unit 117 and a higher-level controller 118. Pressure sensors (V/P), position sensors 119 and/or force sensors 120 may be connected to the on-site electronic unit by appropriate signal lines, in particular with the option of multi-variable control as described, for example, in WO94/22655 for a purely electric drive.

[0028] In parallel with line 111, a second line 112 branches off from line 108. Line 112 is connected to a throttle 103. Accordingly, the oil flow passes through line 112 only in the outflow direction to tank 110, as indicated by a single arrow 121. Throttle 103 is also motor-driven by a servo motor 104, so that the above-mentioned control and regulating technology is used accordingly. The output of the throttle 103 leads back into tank 110 via a drain line 122. The oil flow into chamber 109 is produced by pump 30, but the outflow may be accomplished either only by the pump, or for the high-speed phases of the casting piston, it may be accomplished by both means, i.e., by pump 30 as well as throttle 103. On the back side of the piston, preferably a gas, especially nitrogen gas (N₂) is proposed, this gas being stored in an accumulator 100. The back side of the piston may also be acted upon by oil, in which case a piston-type accumulator is situated between the back side of the piston and the gas pressure reservoir.

[0029] Between the back side 14 of the piston 15 and the pressure reservoir 100 there is a shot valve 101 and a connecting line 102 so that an injection cycle can be initiated and coordinated via a control line 123 and the higher-level controller 118.

[0030]FIG. 9 shows an example of the structural embodiment of the throttle 103 with a servo motor 103 directly flange-connected to it. The main part of throttle 103 is a longitudinally displaceable control piston 130 which opens or closes, blocking the flow between line 112 and the outflow line 122, depending on the position of the connecting chamber. The flow is blocked in the position shown here. If the control piston 130 moves to the left, a more or less large throttle gap is established between the pressure side and/or the inflow chamber 122 and the connecting chamber 131. The exact adjustment of the throttle gap is made by a ball spindle 133, which is mounted in a nut 134 in the control piston 130. A rotational movement of the ball spindle 133 clockwise or counterclockwise according to arrows 135 and 136, respectively, yields a displacement of the control piston 130 in the longitudinal direction according to arrow 137 in throttle housing 138. The spindle is connected directly by a coupling 138 or an intermediate gear to the shaft of servo motor 104.

[0031]FIG. 10 shows a preferred control and regulating technology in the sense of a multi-axial drive with three servo motors M1, M2, M3 that are controllable and regulatable independently and, if necessary, chronologically. S1, S2 and S3 denote control signal lines and R1, R2, R3 denote position acknowledgment lines for each axle.

[0032]FIG. 11 shows various injection profiles for position, speed and force. The enormous increase in speed in the range from 450 to 500 ms is noteworthy. This diagram shows on the ordinate the injection pressure and/or injection speed curve with the maximum=100% and on the abscissa the time axis in milliseconds. In the example shown here, the throttle operates only for 20 to 30 ms. The pump may be relatively small. It is thus possible to save on the order of ⅔ to ¾ of the energy consumption in comparison with the state of the art. This yields a practical injection rate at a relatively slow forward flow of 0.5 to 1.0 m/sec with the pump alone and with rapid injection or with a rapid shot of 5 to 10 m/sec with both the pump and throttle valve.

[0033]FIG. 12 illustrates the application of this novel invention with a press. 

1. A method for controlling/regulating the linear pressing/casting movement by means of a driving piston on injection molding machines and compression molding machines and presses, whereby a hydraulic medium is used at least on the working side and/or the piston rod side, characterized in that the pressing/casting movement is controllable/regulatable with respect to an oil flow and/or an oil pressure on the piston rod side by means of a pump (30) and at least in phases additionally through a throttle valve (34, 46) that operates in parallel with the pump (30).
 2. The method according to claim 1, characterized in that the piston is driven for the forward movement by the gas pressure, in particular nitrogen (N₂) from a nitrogen container through appropriate valves.
 3. The method according to claim 2, characterized in that the driving piston is operated on both sides with a hydraulic medium and the forward movement is operated by a piston-type accumulator.
 4. The method according to one of claims 1 through 3, characterized in that the pump (30) is designed as a fixed-volume pump and the controlling/regulating of both the pump (30) and the throttle valve is accomplished by a servo motor (104, 105).
 5. The method according to one of claims 1 through 4, characterized in that both the pump (30) and the throttle return flow are connected to an oil container (110), preferably closed by a film covering, and vacuum degassed oil is used.
 6. The method according to one of claims 1 through 5, characterized in that in the hot chamber method, the rapid shot is controllable/regulatable with a pump (30) and throttle valve (34, 46), and in injection molding the rapid injection is similarly controllable/regulatable.
 7. The method according to one of claims 1 through 6, characterized in that in the case of maximum oil flow through the pump and throttle valve according to 100%, the pump (30) is designed for 10% to 50%, preferably 15% to 30% of this oil flow.
 8. A drive system for controlling/regulating a driving piston for the pressing/casting movement in injection molding machines and compression molding machines or presses, whereby a hydraulic medium is provided at least on the working side and/or the piston rod side, characterized in that both a pump (30) and in addition at least one throttle (103) connected in parallel are provided on the piston rod side such that both the flow through the pump (30) and the flow through the throttle (103) are controllable/regulatable by motor means at least for the range of the rapid forward movement.
 9. The drive system according to claim 8, characterized in that the driving piston (15) has a gas impingement with a direct connection to a pressurized nitrogen container on the back side (14) (for the forward movement).
 10. The drive system according to claim 8 or 9, characterized in that the pump (30) is designed as a fixed-volume pump with a servo motor drive (104, 105).
 11. The drive system according to one of claims 8 through 10, characterized in that the throttle (103) has a motor positioning drive, in particular a servo motor (104, 105), the overdrive being accomplished by a transmission, a spindle or a crank drive.
 12. The drive system according to claim 11, characterized in that the positioning drive has a spindle overdrive with a spindle nut whereby the spindle nut is installed in the servo valve, and the servo motor (104, 105), the spindle and throttle (103) form one structural unit.
 13. The drive system according to claim 12, characterized in that the throttle (103) is designed as a piston valve, and the throttle cross-section is adjustable directly by way of the servo motor (104, 105) and the spindle overdrive through a linear displacement.
 14. The drive system according to one of claims 8 through 13, characterized in that it has pressure gauges for measuring the injection force and a distance measuring system for the position of the casting piston.
 15. The drive system according to one of claims 8 through 14, characterized in that it has a control/regulating unit which is designed as a multi-axial and/or multi-variable regulating unit whereby the position and/or the speed and/or the force of the driving piston (15) is/are detectable in particular. 